Sophisticated high performance antennas are commonly used in both commercial and military applications. With respect to commercial applications, high performance antennas are used, for example, to support high speed voice, data, and video communications. In military applications, high performance antennas are, for example, used in conjunction with satellite communications; unmanned aerial vehicles; and various aircraft, ship and ground vehicle missions.
Equally sophisticated antenna measurement systems are used in the development, manufacturing, and maintenance of these high performance antennas. Well known near-field antenna measurement systems provide a convenient method for testing or otherwise measuring the performance of these antennas. Near-field testing is typically conducted indoors, in a relatively small, confined space. The testing equipment generally includes, among other things, a an RF probe antenna (hereafter “probe”) that is scanned over an arbitrary, geometric surface surrounding, or adjacent to, the device (e.g., antenna) under test (DUT). The distance or range between the DUT and the probe may be very short, even to the point where the probe nearly touches the DUT. During the testing, near-field voltage data (both phase and amplitude) is collected as the probe moves over or past each of a plurality of discrete measurement points which lie on and define the aforementioned surface. The near-field voltage data is then transformed into far-field data using well-known Fourier techniques. The resulting far-field data can then be displayed or otherwise used to assess the conventional far-field performance for the DUT without having to make actual far-field measurements.
The near-field measurements must be extremely precise because small errors in the near-field measurements translate into large errors when the near-field data is transformed into far-field data. Thus, any variance in the actual position of the probe and each of the plurality of discrete measurement points will result in significant errors when the near-field data is transformed into far-field data. Prior art systems have focused on two general approaches in an attempt to minimize near-field data measurement errors.
The first prior art approach involves attaching the probe to equipment that is made of strong, rigid material; material which is not susceptible, for example, to mechanical oscillations when moved or material deformation due to wear and tear over time or changes in environmental conditions, such as thermal drift within the testing chamber. Near-field measurement systems that employ such equipment, in combination with highly precise position control systems, are able to better insure that the probe is positioned on or extremely close to the each of the plurality of discrete measurement points when each corresponding RF measurement is made. The problem with this approach is that such measurements systems are extremely costly, and still some error can still be expected.
The second prior art approach generally involves calibrating the test equipment in advance of capturing near-field data measurements. For example, U.S. Pat. No. 5,419,631 describes a measurement system which compensates for probe position errors due to thermal drift. More specifically, these position errors are identified by a three-axis motion tracking interferometer apparatus that performs a distance measurement between the DUT and the probe at the specified measurement points. These distance measurements occur periodically throughout the RF data collection. Then, during post processing a simple phase correction based on frequency and change in distance measured at a few points is applied to the plurality of measurement points. The problem with prior art systems such as this is that the process of collecting the distance measurements for purposes of calibration takes a significant period of time and, moreover, any variance in the conditions when the calibration measurements are taken compared to when the RF measurements are taken, will cause the calibration data to be inaccurate, leading to inaccuracies in the near-field data and, more significantly, inaccuracies when the near-field data is transformed to far-field data. A further limitation is that this simple phase correction in effect results in a single pointing direction correction rather than a plurality of pointing directions.
Accordingly, better systems and methods are needed to provide high speed, accurate near-field RF measurements, particularly for electrically large antennas, without the need for extremely expensive equipment and/or materials. These systems and methods must also effectively measure a wide variety of modern high performance antennas using any standard near-field techniques including planar, cylindrical, or spherical techniques.